Technical Field
This document relates to an atlantoaxial joint replacement device and method for restoring stability while allowing axial rotation.
Background
The atlantoaxial joint includes the first and second cervical vertebrae (C1-C2 vertebrae) and their intervening ligaments. This joint of the spine is unique in that it has no intervertebral disc, with the primary articulating surfaces being the left and right facet articulations. The atlantoaxial joint allows an extremely large amount of axial rotational motion (˜90° bilateral). This quantity of motion is by far the most of any single level in the spine, and accounts for more than half of the axial rotation of the entire neck. Such large motion is allowed by the pivoting of C1 about the vertically oriented odontoid process (dens) of C2. This motion is similar to rotation of a wheel on its axis, hence C1 is also referred to as the “atlas” and C2 is also referred to as the “axis”.
Throughout the entire range of motion during left and right axial rotation at C1-C2, the axis serves as a fixed center of rotation. This joint is therefore more hinge-like than other joints of the spine where the center of rotation is not necessarily fixed, but instead may shift to different positions at different phases of motion. C1 is kept centered on the axis by being sandwiched between the anterior bony arch of C1 anteriorly, and the horizontal cruciate ligament (transverse atlantal ligament) posteriorly.
Injury or other pathological occurrence can make C1-C2 unstable. In such instances, risk of neurological injury is high. Some injuries that destabilize the C1-C2 joint are surgically treatable, such as fracture of the dens. In this particular case, a screw may be used to repair the fracture, reattaching the dens peg to the body of C2 so that the hinge-like joint is again functional.
However, most other injuries to C1-C2 are treated instead by fusion. Typically, for fusion, the surgeon attaches screws and rods posteriorly to immobilize C1-C2 and places or wires a bone graft in apposition to the surfaces of C1 and C2 so that eventually the level becomes fused. However, such surgery has the consequence that more than half of the patient's neck rotation is restricted.